1. Field of the Invention
This invention is related to hydro-metallurgical processes for the recovery of precious metals from their ores, and in particular, to the recovery of precious metal values from ores, concentrates, tailings and other products of mining and metallurgical operations.
2. Description of the Prior Art
Precious metal values such as gold and silver may be extracted from their ores by a leaching process. The ores may be specifically mined for their gold or silver content, or the metals may be recovered from tailings or other by-products when a base metal, such as lead, zinc, and copper are refined. The term "precious metal value" is generally understood to refer to gold, silver, platinum and the metals of the platinum group, including osmium, ruthenium, iridium, palladium and rhodium.
According to conventional practice, precious metal values are recovered from their ores by smelting or by a cyaniding process in which the ore is treated with cyanide compounds, usually in an alkaline medium, to complex out the precious metals. The precious metal values are then separated from the cyanide solution by precipitation of the cyanides or by addition of a carbonaceous, sorbent material followed by floatation and reduction of the cyanide concentrate.
The use of the cyanide leaching process has been limited because of the poisonous nature of the cyanide compounds. The practice of open air spraying of tailings with a cyanide solution, while economical, is harmful to the environment, in particular to wildlife and natural water supplies. Moreover, the open air lixiviant spraying of tailings is relatively inefficient because of the relatively large volume of lixiviant solution required and because the exposed surface area per unit volume of tailings is relatively small.